Lithium secondary batteries, a type of nonaqueous batteries, are characterized by their high energy density, and thus are used widely as power sources for portable devices such as portable phones and notebook personal computers. Furthermore, by taking advantage of their high energy density characteristics, studies on use of lithium secondary batteries as power sources for vehicles, such as electronically assisted bicycles, electric motorcycles, electric vehicles and hybrid vehicles, have been conducted in recent years. Since such power sources for use in vehicles are larger in capacity than power sources for portable devices, it is important to ensure further safety of the batteries.
In currently available lithium secondary batteries, polyolefin-based microporous films (microporous membranes) having a thickness of, for example, about 20 to 30 μm are used as separators for being interposed between positive and negative electrodes. Of polyolefins, materials having a low melting point such as polyethylene may be used as separator raw materials in order to ensure a so-called shutdown effect. The shutdown effect improves the safety of a battery in the event of shorting or the like by allowing the constituent resin of the separator to melt at a temperature equal to or lower than the thermal runaway temperature of the battery to close the pores, thereby increasing the internal resistance of the battery.
By the way, uniaxially- or biaxially-oriented films are used as these separators for increased porosity and to improved strength, for example. Since such separators are provided as independent films, they are required to have a certain level of strength in view of workability, etc., and the drawing ensures their strength. In such oriented films, however, the degree of crystallinity is increased, and the level of the shutdown temperature is also increased close to the thermal runaway temperature of the battery. Thus, it is hard to say that the margin for ensuring the safety of the battery is adequate.
Moreover, the films have been distorted as a result of the drawing and may shrink due to residual stress when they are subjected to high temperatures. The shrinkage temperature is very close to the melting point, namely, the shutdown temperature. Therefore, when using such a polyolefin-based microporous film separator, the current must be reduced to prevent a rise in the temperature of the battery as soon as the temperature of the battery reaches the shutdown temperature in the event of, for example, abnormal charging. If the pores cannot be closed adequately and the current cannot be reduced right away, the temperature of the battery can elevate easily to the shrinkage temperature of the separator, so that internal shorting may pose the danger of ignition.
As a technique for improving the reliability of a battery by preventing such shorting resulting from thermal shrinkage of a separator, for example, it has been proposed to use a separator including a highly heat-resistant porous base material, filler particles and a resin component for ensuring the shutdown function to form an electrochemical device (Patent Documents 1 to 3).
It has been also proposed to improve the heat resistance of a polyolefin porous film by forming on the film a heat-resistant layer predominantly composed of a heat-resistant resin and inorganic fine particles (Patent Documents 4 to 6).
According to the techniques disclosed by Patent Documents 1 to 6, batteries having an excellent level of safety in which thermal runaway is less likely to occur even in the event of abnormal overheating can be provided.